float slope(float x)
{
int i,p;
float slope;
for (i=0;i<1000;i++)
	{
	if(points[i][0]>x)
                {
                p=i; break;
                }
        }
slope = ((points[n+1][1]-points[n][1])/(points[n+1][0]-points[n][0]) + (points[n][1]-points[n-1][1])/(points[n][0]-points[n-1][0]) )/2  
return slope;
}    
         
        


